This invention relates to the electric field absorption type or Electro Absorption (EA) optical modulator and the optical transmission system that applies to the optical transmitter which drives the Distributed Feed-Back (DFB) laser and the accumulated EA/DFB integration optical source.
The development of optical transmitters for high-speed optical transmission has experienced tremendous growth in the information communication market in recent years. The fiber loss becomes the smallest to develop a relay distance if a laser beam is in the 1.5 μm band in the optical source or the optical element of the optical transmitters. However, the most of existing fibers are usual dispersion fibers which have the group velocity dispersion of 0 in the band light of 1.3 μm. For this reason, the group velocity distributing by the usual dispersion fibers is large in the band light of 1.5 μm.
The large group velocity dispersion is caused by chirping that is one of the characteristics of the optical transmitter. Due to chirping, the transmission degradation of the optical pulse occurs. Optical amplifiers are used in long-range transmission, and the waveform degradation by the non-linear effect of the optical fibers occurs.
The chirping is frequency flicker which accompanies the optical amplitude modulation. To carry out high quality high-speed optical transmission along the usual dispersion fiber line, it is important to optimally reduce the chirping of the optical transmitter and to consider the non-linear effect of the optical fiber in the implementation on circuits. IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 8. No. 7, JULY in 1996 in page 944-946.
For an over-10 G bit/s-high-speed optical transmitter with the usual dispersion fiber line, the method of externally modulating low chirping characteristic is better than the method of directly modulating, and the method of external modulation must be adapted. Especially it is observed a laser optical source is carefully integrated. The external modulation is integrated with the EA/DFB optical source for practical use.
The merit of using the EA/DFB integration optical source includes the reduction in an optical transmitter size and the electric power consumption. For example, the merit is described: Electronic Information Communication Society, Communication rally B-10-91 in 1999 and IEEE GLOBECOM1996 CONFERENCE RECORD, page 1916-1919. The EA/DFB optical source is used as a favorable optical source or the optical element in the high-speed Wavelength Division Multiplexing (WDM) system as well as the Time Division Multiplexing (TDM) system. These WDM and TDM systems are extensively studied and developed in recent years.
The chirping of the EA optical modulator or the EA/DFB integration optical source is characterized by the refraction ratio variation and the optical absorption coefficient variation. The refraction ratio is called alpha parameter. The close relation exists between the alpha parameter and the fiber transmission characteristic. Consequently, it is important to control the fiber transmission characteristic of the optical transmitter that is equipped with the EA optical modulator or the EA/DFB integration optical source to control the alpha parameter for optimizing the performance.
FIG. 1 shows a measurement result that is the alpha parameter (“a”) of the EA optical modulator. The normalized optical output ratio characteristic is also shown with respect to the applied voltage (“Vea”) to the EA optical modulator. From the figure, it is understood that the alpha parameter and the normalized optical output ratio characteristic are dependent on the EA optical modulator applied voltage. It is possible to control the normalized optical output ratio by controlling the bias voltage and driving the amplitude of vibration with the EA optical modulator driving voltage.
There is a method of compensating the alpha parameter of the EA optical modulator set in to fit the EA modulator in any optical transmission system as well as to stabilize the EA optical modulator characteristic.
FIG. 2 shows a transmitter sample as shown in JP11-119176. A driving part 8 drives an EA optical modulator 4 in response to the vibration amplitude of a bias and amplitude controller 9. The EA optical modulator 4 modulates output light from an optical element or optical source 5 and outputs modulator output light 7. The optical output power of the optical source 5 controls the back-face output light from the optical element 5 by a photo diode or photo diode (PD) 6. The modulator output light is stabilized by an auto power control (APC) 11 which compensates for the difference between the back-face output light and a feed back to the current source 10.
Still referring to FIG. 2, the light of the optical element 5 is absorbed by the EA optical modulator 4 and is changed into the photoelectric current Iph. The amount of Iph changes according to the optical output power of the optical element 5 and a voltage level applied to the EA optical modulator 4. The applied voltage level to the EA optical modulator 4 and the modulator output light 7 are kept constant by monitoring the amount of Iph at an electronic absorption monitor 25 and by feeding back to the driving part 8 through the bias and amplitude controller 9.
FIG. 3 shows an example of the transmitter in JA9-179079. In the figure, the modulator output light from the EA optical modulator is divided by an optical coupler 15. One output becomes the modulator output light 7 while the other one is inputted into a photo diode (PD) or optical detector 16 to indicate a change in the optical output power and to control a current source 10 via a calculating part 12 for maintaining a constant optical output power level to an optical element 5.
Next FIG. 4 shows an example of the transmitter in JP6-152043. In the figure, two EA optical modulators 4 and 17 are respectively located at either side of an optical element 5. The modulator output light 7 is kept constant by feeding the change amount in the back-face optical output from a current source 10.
Consequently, when the alpha parameter is controlled to improve the fiber transmission characteristic of the optical transmitter, a driving point at which the EA optical modulators 4 and 17 are driven is changed and the optical output of the optical transmitter is also changed. Generally, it is difficult to adjust the optical transmitter to achieve desired characteristic specifications since the fiber transmission characteristic and the optical transmitter output cannot be determined at the same time.
In the transmitter as shown in FIG. 2 it is possible to keep constant the fiber transmission characteristic that is affected by the change in the electronic absorption as indicated by Iph. However, since the electrical current source 10 is not controlled by the change of the electronic absorption, the optical output power is changed.
Iph is changed for compensating the change in the optical input power to the EA optical modulator 4 from optical element 5 or the driving point of the EA optical modulator 4. The applied voltage level to the EA optical modulator 4 is also changed at the same time. As a result, the alpha parameter is changed, and the fiber transmission characteristic is thus changed. Moreover, the above described steps are taken when the optical output changes due to degradation in the optical elements.
In an example shown in FIG. 3, the current in the current source 10 is adjusted based upon the change in the monitored optical power from an optical detector on PD 16. But, as the driving point for the EA optical modulator 4 in the driving part 8 changes, the alpha parameter dose not maintain a constant value. Also as one of the modulator output light is divided by the optical coupler 15, the loss of the modulator output for the transmission in the fiber becomes a substantial problem. It is undesirable to set the optical coupler 15 for dividing the optical output, and the costs of and space for other optical parts are prohibitive.
For example, as shown in FIG. 4, because the change in the output light from the optical element 5 changes the EA optical modulator driving point in the driving part 8, the alpha parameter does not remain constant either. Also, the two EA optical modulators 4 and 17 must have the same normalized optical output ratio characteristic. These EA optical modulators 4 and 17 have to have the same optical axis of the optical element 5 onto the identical chip. For this reason, it is difficult to make an integrated EA/DFB optical source.